U.S. patent number 4,087,654 [Application Number 05/636,297] was granted by the patent office on 1978-05-02 for echo canceller for two-wire full duplex data transmission.
This patent grant is currently assigned to Bell Telephone Laboratories, Incorporated. Invention is credited to Kurt Huggo Mueller.
United States Patent |
4,087,654 |
Mueller |
May 2, 1978 |
Echo canceller for two-wire full duplex data transmission
Abstract
An adaptive echo canceller for digital data transmission systems
permits full duplex, i.e., simultaneous bidirectional transmission,
operation at full bandwidth over two-wire transmission facilities.
A transversal filter arrangement digitally synthesizes a
cancellation signal for unwanted leakage, i.e., echoes, through
hybrid junctions directly from the digital data input symbols,
rather than from the analog transmitter output. An error control
signal for correlation with tap signals on the transversal filter
is derived from the output of the receiver, instead of its
input.
Inventors: |
Mueller; Kurt Huggo (Holmdel,
NJ) |
Assignee: |
Bell Telephone Laboratories,
Incorporated (Murray Hill, NJ)
|
Family
ID: |
24551288 |
Appl.
No.: |
05/636,297 |
Filed: |
November 28, 1975 |
Current U.S.
Class: |
379/406.08;
370/291 |
Current CPC
Class: |
H04L
5/1423 (20130101); H04B 3/23 (20130101) |
Current International
Class: |
H04L
5/14 (20060101); H04B 3/23 (20060101); H04B
003/24 () |
Field of
Search: |
;179/170.2,170.6,170.8
;178/58R ;333/18 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Koll and Weinstein; "Simultaneous Two-Way Data Transmission Over a
Two-Wire Circuit"; I.E.E.E. Transactions on Communications, vol.
Com-21, No. 2; Feb. 1973; pp. 143-147. .
"Application of Automatic Transversal Filters to the Problem of
Echo Suppression"; Bell System Technical Journal; vol. 45, No. 2;
Dec. 1966; pp. 1847-1851. .
"An Adaptive Echo Canceller"; Bell System Technical Journal; Mar.
1967; vol. 46, No. 3; pp. 497-511..
|
Primary Examiner: Claffy; Kathleen H.
Assistant Examiner: Myers; Randall P.
Attorney, Agent or Firm: Kearns, Jr; Joseph P.
Claims
What is claimed is:
1. An echo canceller for digital data transmission systems
including terminals having both a transmitter section and a
receiver section for simultaneous two-way signaling at full
bandwidth over a common two-wire signal path comprising at each
terminal
an adjustable signal processor for synthesizing an
echo-cancellation signal having its input connected to a data
symbol source preceding the transmitter section for an outgoing
signal toward said two-wire signal path and its echo-cancelling
output signal connected in subtractive relationship with the
incoming signal over said two-wire signal path from a remote
terminal to form a substantially echo-free signal for said receiver
section and
means responsive to the error difference between quantized and
actual outputs of the receiver section for generating a control
signal causing said echo-cancellation signal from said signal
processor to minimize said error difference.
2. The echo canceller defined in claim 1 in which said adjustable
signal processor comprises a synchronously tapped delay medium,
an adjustable gain device for each tap on said delay medium,
correlator means for each tap jointly responsive to the control
signal from said generating means and to the data samples at each
delay-medium tap for updating said adjustable gain devices, and
means for combining tap signals operated on by said adjustable gain
devices.
3. The echo canceller defined in claim 1 in which said adjustable
signal processor comprises
a serial memory providing access to input, output and intermediate
taps,
an exclusive-OR gate assigned to each tap on said serial memory for
correlating the data samples on said serial memory with said
control signal,
an adjustable gain device for each tap of said serial memory,
and
means for updating said gain devices under the control of said
exclusive-OR gates.
4. The echo canceller defined in claim 1 in which said transmission
system operates at passband between terminals and the output of
said signal processor is up modulated to such passband before being
subtracted from the incoming signal from a remote terminal.
5. In a two-way data transmission system having four-wire to
two-wire bridges between a common transmission link and terminals
including separate transmitter and receiver sections,
a compensation circuit for transmitter signal components leaking
across said bridge between transmitter and receiver sections at
each terminal for said data transmission system comprising
means jointly responsive to data signals applied to the transmitter
section and an error difference in recovered data signals from the
receiver section for generating a compensation signal,
means for subtracting the compensation signal from said generating
means from data-containing signals incoming from said transmission
link to said receiver section, and
means responsive to an error difference between quantized and
actual outputs of said receiver section for adaptively adjusting
said generating means to minimize said error difference.
6. The data transmission system set forth in claim 5 in which said
generating means comprises a plural tapped transversal filter.
7. The data transmission system set forth in claim 5 in which the
error difference between quantized and actual outputs of said
receiver section are correlated with a plurality of consecutive
data signals applied to said transmitter section and said plurality
of consecutive data signals applied to said transmitter section are
selectively weighted in accordance with accumulated error
correlations and combined to form said compensation signal.
8. The data transmission system set forth in claim 5 in which said
generating means comprises a transversal filter having a plurality
of signal taps spaced by the synchronous data signaling
interval.
9. The data transmission system set forth in claim 5 in which said
generating means comprises
a serial memory having signal taps at input, output and
between-stage positions,
an exclusive-OR gate for correlating the polarity of the error
differences between quantized and actual outputs of said receiver
section with data signal samples at each tap on said serial
memory,
an adjustable multiplier for selectively weighting data signal
samples at each tap on said serial memory in accordance with
accumulated outputs of said exclusive-OR gates, and
a signal combiner for all the selectively weighted outputs of said
multipliers for forming said compensation signal.
Description
FIELD OF THE INVENTION
This invention relates to the suppression of echo and leakage
currents from digital data transmitted and received through hybrid
junctions in two-way telephone transmission systems.
BACKGROUND OF THE INVENTION
A data system using full duplex transmission possesses a number of
advantages over one restricted to half-duplex operation. Full
duplex transmission means simultaneous full bandwidth transmission
in both directions over a common medium. Half duplex transmission
means alternate full bandwidth transmission in the two directions.
With full duplex operation start-up and turnaround delays are
avoided, while half duplex operation these delays are inevitable
and become very wasteful of transmission time when turnaround delay
times are comparable to message block lengths. For interactive data
terminal operations full duplex transmission is essential. In the
past full duplex transmission was generally based on the use of
private line telephone channels with four-wire facilities, i.e.,
with separate, isolated pairs for each direction of
transmission.
For full duplex operation on two-wire facilities, such as are
generally available on the public switched telephone network, it
has been necessary to split the single available transmission
channel into high and low bands dedicated to particular
transmission directions. Only half the available bandwidth can then
be used for each transmission direction to the detriment of the
transmission rate.
Oftentimes it is desirable to be able to employ switched-network
two-wire telephone channels as back-up for, or dial-in access to,
private line systems.
Many long-haul toll telephone facilities include echo suppressors
which are designed to suppress reverse traffic when forward traffic
has seized the facility even though long-haul facilities are
generally four-wire arrangements. Whenever the direction of traffic
is to be reversed, one set of echo suppressors must be disabled and
another set activated. Thus, simultaneous two-way traffic is
precluded without special arrangements for disabling all echo
suppressors, but such arrangements are routinely included in many
full-duplex voice grade modems.
For simultaneous two-way transmission within the same frequency
band it is mandatory to separate the local transmitter signal from
the usually weak signal received from the remote site. Hybrid
networks or bridge circuits, realizable with or without
transformers, are standard and well-known arrangements for
achieving this separation. In such circuits, a terminating
impedance equal to the impedance of the two-wire line must be used
for perfect separation. Due to the complex and frequency dependent
nature of this impedance, only a very approximate compensation is
possible in practice. Direct leakage across the hybrid and delayed
echoes caused by signals reflected on more distant line impedance
mismatches will thus cause transmitter signal components to
interfere with the received distant signal. The effect of delayed
echoes is particularly annoying in communication over satellite
channels.
Adaptive echo cancellers implemented by transversal filters have
been proposed for analog facilities by, for example, J. L. Kelly,
Jr., and B. F. Logan, Jr. in U.S. Pat. No. 3,500,000 issued Mar.
10, 1970. In the latter echo canceller a portion of the analog
signal incoming to a hybrid junction on the four-wire side is
passed through a transversal filter with adjustable tap gain
controls to synthesize a cancellation signal for subtraction from
the signal outgoing from the hybrid junction. The resultant
outgoing signal is clipped and correlated with the sequence of
samples of the incoming signal appearing at the taps of the
transversal filter to form control signals for the tap gains or
weighting coefficients of the transversal filter.
A similar arrangement is described by F. K. Becker and H. R. Rudin
in the Bell System Technical Journal [Vol. 45, 1966, pp.
1847-1850], in a paper called "Application of Automatic Transversal
Filters to the Problem of Echo Suppression". Results are achieved
with a practical realization are reported by V. G. Koll and S. B.
Weinstein in the IEEE Transactions on Communications, [Vol. COM-21,
No. 2, 1973, pp. 143-147] in a paper entitled "Simultaneous Two-Way
Data Transmission Over a Two-Wire Circuit." The transversal filter
as applied to automatic and adaptive equalization of digital data
signals has been disclosed in R. W. Lucky U.S. Pat. No. Re-27,250
as comprising a delay line having a plurality of taps equally
spaced at T-second intervals corresponding to the reciprocal 1/T of
the synchronous data symbol rate.
In order to implement such an echo canceller for analog voice
signals large numbers of taps are required on the transversal
filter spaced by the reciprocal of twice the highest frequency to
be expected in the echo or leakage signal and spread over the
maximum echo delay. For voice channel bandwidths the number of taps
required is then on the order of eight for each millisecond of
round-trip echo delay. Furthermore, two multiplications are needed
for each tap, one for correlation purposes and tap gain
adjustments, and another for computation of the transversal filter
output signal. Even with digital techniques using analog-to-digital
and digital-to-analog conversion, the circuit complexity of this
approach becomes prohibitive if distant echos have to be
cancelled.
It is an object of this invention to provide simultaneous full
duplex digital data transmission over two-wire communications
facilities with full bandwidth utilization for each transmission
direction.
It is another object of this invention to adapt the transversal
filter to echo and leakage cancellation in two-way data
transmission systems to permit full bandwidth, full duplex
operation.
It is a further object of this invention to reduce the cost and
complexity of echo and leakage cancellers for data transmission
systems.
SUMMARY OF THE INVENTION
In accordance with this invention the data-like nature of the
echoes or leakage components from transmitter to receiver through
the four-wire to two-wire junction in a two-way data terminal is
utilized to derive a cancellation or compensation signal directly
from the data symbols. The echo canceller for data modems which
include a bridging connection to a two-wire transmission channel
comprises a processor taking baseband data symbols directly from
the data source (with or without randomizing) before modulation,
shaping, or filtering; an error control circuit for the processor
which takes an input from the receiver after the incoming analog
signal has been converted into baseband data symbols; and a summing
circuit for combining the output of the processor with the incoming
received signal. The processor preferably comprises a linear
sequential transversal filter with taps spaced at the data symbol
transmission interval, an adjustable gain device at each tap, a
correlator at each tap for a common error signal and the data
symbol thereat, and a summer for tap signals selectively weighted
by the gain devices. The error control signal is derived by
comparing the actual and quantized outputs of the data receiver.
The correlation of the error signal with the respective tap signals
results in a mean square minimization of the residual echo or
leakage component. Effectively the mean-square error difference
between each tap output and its contribution to the overall error
output is forced toward zero. The disclosed echo canceller can be
looked at as an adaptive transmitter, with identical inputs of and
in parallel with, the main transmitter.
Since the data symbols applied to the processor are either one bit
symbols in the case of a binary system, or two or three bit symbols
at most in the case of a multilevel system, the following features
result: the required serial memory size (number of taps) of the
transversal filter is substantially reduced, and the required
multiplying operations for both output computation and tap gain
adjustments are simplified. In the case of two level signaling, the
multiplier circuit can be replaced by a simple adder, since the tap
signals are only .+-.1. Cancellation itself can be done in passband
or baseband, in digital or analog form, and either on a continuous
basis or at sampling instants only.
BRIEF DESCRIPTION OF THE DRAWING
The objects, advantages and features of this invention will become
more apparent from a consideration of the following detailed
description and the drawing in which:
FIG. 1 is a block diagram of an adaptive echo or leakage canceller
for a two-way telephone transmission system according to principles
known to the prior art;
FIG. 2 is a block diagram of an improved adaptive echo or leakage
canceller for a terminal of a digital data transmission system
according to this invention; and
FIG. 3 is a more detailed block diagram of an adaptive echo or
leakage canceller for a terminal of a digital data transmission
system employing a transversal filter according to this
invention.
DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS
The echo canceller disclosed by Kelly et al. in their cited patent
application was intended principally to cancel echoes arising in a
point-to-point analog transmission link in which the terminals were
connected by long-haul four-wire facilities over which very long
delays were experienced. The echo canceller operated on the signal
on the incoming four-wire leg as it entered the hybrid junction and
applied the correction signal to the outgoing four-wire leg as it
left the hybrid junction. Effectively the echo canceller was in
parallel with the hybrid junction on the four-wire side and both
its input and output were analog in form.
FIG. 1 shows the structure of an echo or leakage canceller for a
digital data transmission system designed in accordance with the
principles of Kelly et al. The data transmission system depicted
comprises a west terminal to the left of two-wire line 10, an east
terminal to the right of two-wire line 10 and two-wire line 10. The
west terminal comprises a transmitter 11, receiver 12, a processor
13, error control 14, difference amplifier 15, hybrid junction 16
and balancing network 17. Similarly, the east terminal comprises
transmitter 21, receiver 22, processor 23, error control 24,
difference amplifier 25, hybrid junction 26 and balancing network
27. The east and west terminals are mirror images of each other
with respect to two-wire line 10.
An intelligence signal from transmitter west 11, whether in
baseband or passband form is applied to one input of hybrid
junction 16 and also is tapped off to processor 13. At the same
time an incoming signal on two-wire line 10 incident at hybrid
junction 16 is intended to be delivered to receiver west 12 without
contamination by any outgoing signal originating in transmitter
west 11. If hybrid 16 were perfectly balanced by network 17, there
would be no such contamination. However, network 17 is fixed in
value and can provide the ideal termination for only one line
condition. The latter, however, is dynamic and time varying.
Accordingly, a certain unavoidable portion of the outgoing signal
from transmitter west 11 leaks across hybrid junction 16 and joins
as an echo with the received signal. The received signal component,
further corrupted by noise, is normally applied directly to
receiver west 12. However, as part of the echo canceller,
difference circuit 15 is interposed between hybrid junction 16 and
receiver west 12. Difference circuit 15 is also supplied with the
output of processor 13, which ideally generates a compensation
signal exactly matching the leakage signal across hybrid junction
16.
The above description applied in a straightforward manner to the
east terminal.
FIG. 2 illustrates in block diagram form the improved echo
canceller for digital data transmission systems. Only the west
terminal is shown, since the east terminal is a mirror image of the
former. FIG. 2 differs from FIG. 1 in explicitly depicting data
source 30 and data sink 40, whereas FIG. 1 was implemented for
analog signals appearing at the input and output of the hybrid
junction. FIG. 2 comprises a processor 33 and error control 34
directly responsive to the respective output of data source 30 and
input of data sink 40 on the customer side of transmitter 31 and
receiver 32, instead of on the hybrid junction side as in FIG. 1.
Difference circuit 35 is in the same relative position with respect
to hybrid junction 37 and receiver 32 as their counterparts in FIG.
1; but it is to be understood that echo compensation could also
take place after some of the receiver functions have already been
accomplished; e.g., in a passband system error control 34 and
processor 33 may be designed for compensation in the baseband,
after demodulation of the contaminated received signal.
Moving the echo canceller to the input side of the transmitter and
output side of the receiver permits the computation rate of
processor 33 to be scaled down to the data symbol rate for echo
compensation at sampling instants only, rather than to twice the
rate of the highest frequency component as required in the prior
art.
FIG. 3 is a more detailed block diagram of the echo canceller for
data transmission systems according to this invention which employs
a transversal filter. Again only the west terminal is shown, the
east terminal being understood to be the same.
Data source 50 provides a synchronous succession of data symbols
a.sub.k at T-second intervals (where T is the fixed interval
between data symbols and k is as integer indexing such symbol
intervals), restricted to discrete values, to transmitter 51 which
spectrally shapes the symbols and applies them to the transmission
channel by way of echo path 61. The symbols a.sub.k are also
provided at junction 54 to echo cancellation processor 53 (broken
line box). Return signals from echo path 62, including a leakage
component s.sub.k and the desired signal x.sub.k, are applied to
one input (+) of difference circuit 63, which has a compensation,
or echo-cancelling, signal f.sub.k applied to another input (-).
Desired signal x.sub.k contains data symbols b.sub.k transmitted
from the east terminal. The corrected signal y.sub.k is processed
as necessary in receiver 52 to output signal z.sub.k and detected
by quantizer 64 to furnish data symbols b.sub.k to data sink 60. An
error signal e.sub.k is developed from the difference taken in
subtractor 65 between the continuous input and discrete output of
quantizer 64.
Within processor block 53 is a transversal filter of the type and
structure described in the cited Lucky patent comprising delay
units 55 (two such designated 55A and 55B are shown explicitly in
FIG. 3), correlator 56 (shown explicitly as 56A through 56C
connected respectively at the input of delay unit 55A, the junction
tap between delay units 55A and 55B and at the output of delay unit
55B), and multipliers 57 (shown explicitly as 57A through 57C
connected to the same points on delay units 55 as correlators 56),
and sub-processor 66. Correlators 56 at each tap are coupled to
multipliers 57 thereat by electrical or mchanical means 58
(indicated by dashed lines) so that the output of a correlator 56
increments of multiplier 57 in the appropriate direction. Delay
units 55 can advantageously be binary shift registers storing a
sequence of data samples a.sub.k. Although only a single input, an
intermediate and an output tap are shown explicitly in FIG. 3, it
is to be understood that a larger number will generally be required
in a practical embodiment. Multipliers 57 can be considered to have
gain coefficients c.sub.k which are incremented according to the
outputs of correlators 56 in the appropriate direction as mentioned
above. Correlators 56 are multipliers (exclusive OR gates in the
most simple version) with one input connected in common to an error
control signal on lead 67 from subtractor 65. Substractor 65 is
substantially the same as that shown and described in the cited
Lucky patent as element 32 and can be implemented in an obvious
manner by an operational amplifier. The other input of each
correlator is connected to a tap or junction associated with each
delay unit 55. The summation of the products of the data samples
and the tap-gain coefficients appears at junction 59 to form a
correction signal that is applied to sub-processor 66.
Sub-processor 66 may be a straight wire in the case of a Nyquist
system where compensation is only required at the sampling instants
and where timing at both locations is synchronized. It may be a
smoothing filter when continuous compensation is required. It may
be an up-modulator in the case of a passband system. The output of
sub-processor block 66 forms an echo cancellation signal f.sub.k
for application to a difference circuit 63.
Difference circuit 63 combines the incoming echo path signal
(s.sub.k + x.sub.k) on lead 62 with the echo cancellation signal
f.sub.k from processor 53 to form received signal y.sub.k = x.sub.k
+ s.sub.k - f.sub.k. Echo cancellation will occur when the
compensation signal f.sub.k synthesized in processor 53 is equal to
echo signal s.sub.k. The incoming signal on lead 62 includes the
desired far-end digital signal x.sub.k including data elements
b.sub.k as operated on by the transmission channel impulse response
h(t); the undesired near-end signal s.sub.k reflected across the
hybrid junction including outgoing data elements a.sub.k as
effected by the echo response across the hybrid junction; and noise
n.sub.k.
The cancellation signal f.sub.k is the summation of the products of
the outgoing data elements a.sub.k and the tap-gain multiplying
factors c.sub.k. The difference signal y.sub.k in the output of
difference circuit 63 is filtered (and demodulated in the case of a
passband system) in receiver 5 to form an output signal z.sub.k.
The latter signal is detected or quantized in quantizer 64 to
estimate desired far end received symbols b.sub.k. The error
difference between the continuous output z.sub.k of receiver 52 and
the quantized output b.sub.k is an error signal e.sub.k on leads 67
and 68. The error signal e.sub.k appearing on lead 67 is correlated
with tap samples a.sub.k as previously mentioned. The error signal
e.sub.k on lead 68 is available for control of a transversal
equalizer which may be employed in receiver 52 to minimize
intersymbol interference in the desired component of the incoming
signal x.sub.k.
It can be shown that the correlation of the outgoing tap samples
a.sub.k with the error signals e.sub.k is proportional to the
gradient of the mean square echo error with respect to the tap-gain
weighting coefficients. To remove the constraints of long time
averaging as required for exact correlation, the tap weighting
coefficients are incremented at each sampling instant or
periodically by a constant step size times the product of error and
tap signal (estimated gradients). Thus, the algorithm for adjusting
tap-gain coefficients can be expressed mathematically as:
where
c.sub.n+1 = updated vector of tap-gain coefficients
c.sub.n = present vector of tap-gain coefficients
g = step size
a.sub.n = present vector of tap samples
e.sub.n = present error value.
In an even simpler implementation, the coefficient increments are
fixed in magnitude, and their polarity is determined by the product
of the polarities of error signal and tap sample (exclusive-OR
correlator).
Equation (1) is an adaptive algorithm coverging to a set of tap
gain coefficients that will yield optimum echo compensation. The
value g is selected with due regard to the channel noise, degree of
intersymbol interference and number of taps on the delay medium.
The number of taps in turn is to be determined by the duration of
the echo period.
While this invention has been described in terms of a specific
illustrative embodiment, it is to be understood that it is
susceptible of modification by those skilled in the art to which it
relates within the spirit and scope of the appended claims.
* * * * *